The present invention relates to an automatic transmission for automobiles and, more particularly, to an improvement with regard to a gear means incorporated in the automatic transmission.
An automatic transmission for automobiles generally includes a combination of a fluid torque converter and an auxiliary gear means which provides the direct connection stage as the top gear stage. It is also practiced to incorporate an overdrive means having a reduction gear ratio smaller than unity in an automatic transmission in addition to the aforementioned combination. Conventionally, when an overdrive means is incorporated in an automatic transmission, it has generally been attached to the output side of an auxiliary gear means because such an arrangement requires only a small and easy modification with regard to the entire design of an automatic transmission. However, when an overdrive means is provided at the output side of an auxiliary gear means, the overdrive means must deal with power the torque of which is increased by the auxiliary gear means thereby causing drawbacks such as that heavier gear elements and friction engaging elements are required for the overdrive means which becomes therefore a relatively big and expensive device and yet is still insufficiently durable. Furthermore, the simplicity and ease in modification with regard to the entire design of an automatic transmission available by incorporating an overdrive means at the rear side of an auxiliary gear means is nullified in some cases due to a large modification required for the design of the vehicle body for incorporating such an automatic transmission. In view of these problems, it has been proposed to incorporate an overdrive means between a fluid torque converter and an auxiliary gear means.
As an auxiliary gear means which provides the direct connection stage as the top gear stage, various types of reduction gear mechanisms have been proposed and practiced. These reduction gear mechanisms generally include a planetary gear mechanism wherein the carrier of the planetary gear mechanism is adapted to be selectively connected to another rotary or stationary element by friction engaging means such as a clutch or brake so as to establish various speed stages. Even in an automatic transmission of the aforementioned type including an overdrive means incorporated between a fluid torque converter and an auxiliary gear means, the overdrive means is constructed as a planetary gear mechanism.
A planetary gear means generally comprises a sun gear, a ring gear, a plurality of planetary pinions and a carrier, wherein the planetary pinions are positioned in an annular space defined between the sun gear and the ring gear while meshing with these gears and are supported by the carrier so as to rotate around their own axes as well as to revolve around the sun gear. A plurality of planetary pinions are arranged approximately uniformly or symmetrically in the annular space defined between the sun gear and the ring gear in order to obtain mass balance of the combination of the carrier and the planetary pinions around the central axis of rotation of the assembly. In this connection, if a plurality of planetary pinions are to be arranged perfectly uniformly in the annular space, the following condition must be satisfied: EQU K = (Z.sub.s + Z.sub.r)/N
wherein K is an integer; N is the number of planetary pinions; Z.sub.s is the number of teeth of the sun gear; and Z.sub.r is the number of teeth of the ring gear.
On the other hand, in accordance with the manner of use, the gear ratio (reduction gear ratio) of a planetary gear means is determined by the following formulae: EQU Z.sub.r /(Z.sub.s + Z.sub.r), Z.sub.s /(Z.sub.s + Z.sub.r), Z.sub.s /Z.sub.r
Therefore, when a planetary gear means having a certain required gear ratio is to be designed within the practical range of number of teeth, the aforementioned equation for the uniform arrangement of the planetary pinions is not necessarily satisfied. In fact, in most cases the planetary pinions are not uniformly arranged in the annular space defined between the sun gear and the ring gear.
Conventionally, an unbalance of the mass of the rotary assembly of a carrier and a plurality of planetary pinions caused by such a non-uniform arrangement of the planetary pinions has been compensated by forming a bore or bores in a bridge portion of the carrier so as to remove a part of the mass thereof. However, when a substantial opening or openings are formed in the bridge portion of the carrier, the rigidity of the carrier, particularly that of a part including the bridge portion formed with the openings, is reduced, thereby causing serious damage to the pinion pins, needle bearings, thrust washers or the like which support the planetary pinions, and shortening the lifetime of the automatic transmission. In a conventional automatic transmission which provides the direct connection stage as the top gear stage, the problem of the pinion pins, needle bearings, thrust washers, or the like being damaged often is not very serious because the planetary gear means is locked, so that the planetary pinions do not rotate around the pinion pins, in the direct connection stage which occupies the substantial part of the operating hours of the automatic transmission. However, in an automatic transmission having an overdrive means such as the one of the aforementioned type including a planetary gear means which is interposed between a fluid torque converter and an auxiliary gear means, the planetary gear means makes a planetary movement in the overdrive stage which occupies a relatively large proportion of the operating hours of the automatic transmission, and, because of this, damage caused to the pinion pins, needle bearings, thrust washers or the like due to the reduction of rigidity of the carrier presents a very serious problem.